The
imminent delisting of Honey-buzzard as a scarce migrant in the annual
British Birds reports has great significance for the accepted status
of the Honey-buzzard in Britain. The delisting was reported in Scarce
Migrants in Report on Scarce Migrant Birds in Britain in 2014: Part
I: Non-passerines, British Birds 109(12) 724-748 by Steve White &
Chris Kehoe, including Honey-buzzard account pp.735-736 p.735p.736
and in Abstract p.724.

Relevant
extracts from the report are:

[Main
account: pp.735-736] As the British breeding population slowly
increases – estimated at 25-39 pairs, from which at least 27
young fledged in 2014 (Holling et al 2016) – it becomes
increasingly difficult to maintain any meaningful distinction between
passage migrants, wandering non-breeders and breeding birds. In 2014,
the first occurred in Argyll on 3rd May and in Greater London on 5th,
with spring movements continuing until early June. Autumn passage was
somewhat larger, extending into early October, the last in Sussex on
11th October.

An
unknown number of Scandinavian migrants do presumably occur annually
but a large proportion of the 130-150 birds that have appeared in
recent years (around 60% of them in autumn outside of influx years)
may simply be part of the breeding population, although it is perhaps
just a coincidence that the numbers in autumn each year broadly
reflect, or at least do not exceed, the size of the British
post-breeding population. For this reason we no longer intend to
report on this species in future Scarce Migrants reports, although we
will attempt to collate and publish records for major influxes such
as that in autumn 2000.

[Abstract
p.724] Records of migrant and breeding Honey-buzzards Pernis
apivorus have become increasingly blurred and the species appears
in this report for the last time.

Comments
by NR:

There
are many questions arising from the delisting. So far the credohas been that all Honey-buzzard flying around in
the UK are Scandinavian migrants. That did give some consistency even
if it was plain daft in the light of the physics of broad-winged
raptor migration. Now a brick has been removed from the wall and we
seek a new set of consistent principles, which are likely to
disappoint many people who would rather the Honey-buzzard was
forgotten about altogether!

I'm
not sure the authors have thought through their comments with any
thoroughness. The migrants are reported on a broad front, often to
the north of the official Honey-buzzard population area in southern
England and south Wales. So if they are equating passage numbers with
breeding numbers, it would appear that many migrants are
over-shooting their breeding areas, which is surely unlikely for
experienced adults. There is also the ridiculous assumption that all
passage birds are spotted and identified correctly by birdwatchers
and that all breeding pairs are discovered and known for what they
are. For a bird that migrates on a broad front at considerable
altitude, most Honey-buzzard on passage will be missed by the
recording system. For a secretive breeder, difficult to identify and
nesting in remote areas where few birders go, it is probable that
only a fraction of the nest sites are found. So if the count of
passage birds roughly equals that expected from the breeding
population, it is a coincidence.

It
cannot be presumed that any Scandinavian migrants occur in Britain
annually. Ringing returns, observers on oil rigs, detailed analysis
(by NR) of the 2008 and 2008 movements, the physics of broad-winged
raptor migration, observations of birds resisting drift to W on the
continent, and lack of correlation between continental populations
and movement sizes, all provide no support for this speculation.

What
is very welcome though is the shift in thinking away from the notion
that every Honey-buzzard migrant in Britain is a continental bird.
Seeking a new set of consistent principles is going to be
fascinating, without admitting that the UK breeding population of
Honey-buzzard far exceeds that reported by the RBBP and that the 2000
and 2008 movements are not influxes but just exceptionally visible
passage of British-bred birds, blocked by adverse weather. The recent
Scottish Birds paper discussed elsewhereprovides welcome support for the existence of a
much higher population of Honey-buzzard in northern Britain than
previously suspected by some 'authorities'.

Of
course a vital question is: how many migrants are there across
Britain. The official figures in the Scarce Migrants report show a
mean of 136 from 2010-2014. However, many more are reported on
BirdGuides, where final totals for Honey-buzzard in UK for 2016 are:
April 1, May 45, June 15, July 39, August 79, September 65, October
14, November 0; total 258. These are counts of records not of birds.
Some reports refer to the same locality and others, on the other
hand, involve multiple birds. It's an impressive total, particularly
for a species which is 'so rare' in the UK. All the sightings are
opportunistic by BirdGuides subscribers so IMHO represent a small
proportion of those actually on the move. Nearly all the birds seen
are either UK-breeding or UK-bred: there is no evidence for a
significant fly-over of the UK by continental birds as confirmed in
the Scarce Migrants article. If you assume that only 10% of birds
actually moving are recorded on BirdGuides and make some deduction
for juveniles in September, a UK population is indicated of around
1,000 pairs!

It
is of course also worth questioning the accuracy of the official
totals, which are far below the gross reported numbers. From a
scientific point of view it may well be a triumph of precision over
accuracy: we have a few very nice (precise) descriptions but the
official totals are highly inaccurate when compared to the real
number of birds frequenting the kingdom. In information retrieval
terms, such as a search on Google, the request is so precisely
(tightly) specified that the items retrieved are indeed relevant but
many items are missed, giving low recall: an inaccurate reflection of
the real world. The information on p.302 Separating Common Buzzard
and European Honey-buzzard, by Dick Forsman in Flight
Identification of Raptors of Europe, North Africa and the Middle
East, Christopher Helm (2016), could almost have been written for
bird record committees in Britain who seem to have a thoroughly out
of date approach to identifying Honey-buzzard, particularly the
juveniles, resulting in a high-precision approach that does not even
address the critical feature necessary to identify the species. High
precision inevitably leads to low recall through scientific
trade-offs. This material by Forsman will be reviewed shortly by NR.